[mips] Unified the MipsMemSimm9GPRAsmOperand and MipsMemSimm9AsmOperand operands, NFC.

Summary:
These operands had the same purpose, however the MipsMemSimm9GPRAsmOperand
operand was only for micromips32r6 and the MipsMemSimm9AsmOperand did not
have a ParserMatchClass.

Patch by Scott Egerton

Reviewers: vkalintiris, dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12730

llvm-svn: 247573